A writable or rewritable recording device using an optical recording media, such as CD and DYD drives or Magneto-Optic (MO) drives, has become an important tool for data recording. Every such device includes an optical pickup unit for emitting a laser beam onto a data-recording surface of the optical recording media and detecting the reflected laser beam. The optical pickup unit contains a light source, such as a semiconductor laser device, for emitting the laser beam, and an object lens for focusing the laser beam onto the optical recording media. In addition, the optical pickup unit also contains a driving device for moving the objective lens parallel to its optical axis for correcting focusing aberrations and perpendicular to its optical axis for correcting tracking deviations. The laser beam is thereby focused on the data recording area and follows a recording track of the optical media precisely.
These days most optical pickup units use electromagnetic driving devices to move the objective lens. In an electromagnetic driving device, a focusing coil is used for correcting focusing aberration and another tracking coil is used for correcting the tracking deviations. The focusing coil causes the objective lens to move up or down along its optical axis, while the tracking coil causes the objective lens to move left or right perpendicular to its optical axis.
As the optical media's rotational speed is continuously increased to accelerate its accessing speed, the electromagnetic driving device for correcting tracking deviations and focusing aberrations is also continuously required to have a faster response time and a higher sensitivity. U.S. Pat. No. 5,627,687 discloses an electromagnetic driving device structure whose focusing coil and tracking coil have only up to 30-40% utilization and therefore have an inferior response time and sensitivity. It is usually considered that, to achieve a fast response time and a high sensitivity, using magnets with high magnetism or high-cost, lightweight materials is necessary.